This invention relates to cathode ray tube construction and more particularly to an improvement combination means for expediting improved tube processing and subsequent tube operation.
The progressive development of cathode ray tube technology has included a trend toward the utilization of higher screen potentials along with the miniaturization and compaction of associated electron gun structures encompassed within envelope neck portions of decreased diameters. Consequently, the critically dimensioned spacings between related electrode components in the electron gun assemblies have been reduced in keeping with design parameters. The resultant minimization of these inter-electrode spacings, in conjunction with the high voltage differentials existant within the gun assembly, and the possible presence of contaminants, greatly increases the probabilities of arcing within the tube structure.
It has been conventional practice in cathode ray tube construction to apply a conductive coating on the interior surface of the funnel portion normally extending thereon from the vicinity of the screen into the forward region of the contiguous neck portion. This coating, which usually has a high positive electrical potential applied thereto, via connective means traversing the wall of the funnel, serves as a connective medium conveying that potential to both the screen and the terminal electrode of the related electron gun assembly oriented in the neck portion of the tube. The presence of this high potential increases the possibility of a spark discharge between the terminal electrode and the adjacent lower voltage electrodes in the gun assembly, especially in the presence of aggravating elements such as sublimation deposits, foreign particles, and minute projections extending into the interelectrode spacings. While considerable effort is expended during tube manufacturing to minimize the factors contributive to arcing, the utilization of anode potentials in the order of 30 KV and higher makes the possible presence of arc inducing conditions factors of extreme importance. It has been found that deleterious arcing or dielectric breakdown within the cathode ray tube may exhibit destructive intensities of 100 amperes or more. With the increased employment of solid state components in television and allied display devices, arcing within the cathode ray tube can produce catastrophic effects on the components in the associated operating circuitry of the display device. Additionally, an arc discharge may permanently damage the internal structure in the tube and sublime harmful metallic deposits in the region of the gun structure.
Cleanliness, precision, and care in tube manufacturing are ever-continuing procedures which are conscientiously employed to combat the materializing of conditions conducive for arcing. Nevertheless, human factors in conjunction with processing sublimates and manufacturing tolerances, sometimes combine to produce an undesirable arcing situation. One of the usual practices in tube processing is a step wherein portions of the electron gun assembly are subjected to high voltage conditioning. Such may be exemplarily accomplished by applying a high voltage potential in the order of 40 KV or more between the terminal high voltage electrode of the gun assembly and the adjacently spaced focusing electrode. The intent of this high voltage or sparking condition is to remove potential arcing projections and foreign matter from the inter-electrode spacing.
Another means for combating arcing within a tube has been the discrete usage of high resistance coatings applied to the interior area of the funnel. For example, one such technique is that disclosed by A. V. de Vere Krause in U.S. Pat. No. 2,829,292, wherein a band of resistive coating is internally applied to substantially the juncture region of the funnel and neck portions of the tube envelope to provide a high resistance area to limit the spark discharge current in the region of the electron gun. When employing coatings of this type within the tube, the resistance supplied thereby, being of a value sufficient to limit the current surge in the event of subsequent arcing, additionally limits the power of the high voltage conditioning procedure since the high resistive coating is in the circuit. Therefore, utilization of the resistive coating within the tube in an attempt to reduce subsequent arcing therein, hinders tube processing that is intended to minimize arcing. Since the minimization and elimination of arcing in present-day color cathode ray tubes is assuming ever increasing importance, it is a prime concern in tube manufacturing to achieve an expedient means for adequately controlling the probable arcing environment within the cathode ray tube per se.